Function unix_nanos_to_iso8601_millis 

pub fn unix_nanos_to_iso8601_millis(unix_nanos: UnixNanos) -> String

Converts a UNIX nanoseconds timestamp to an ISO 8601 (RFC 3339) format string with millisecond precision.

Returns the raw nanosecond value as a string if it exceeds the representable datetime range (i64::MAX, approximately year 2262).